This invention relates to alignment interferometers for measuring very small angular displacements.
Known forms of alignment interferometers, also known as autocollimators, utilize beam splitter-reflectors to produce interference fringes. An example of a beam splitter-reflector is a Koester prism which is a compound prism consisting of two identical 30.degree.-60.degree.-90.degree. prisms joined at their longer legs to form a beam-splitting interface. The shorter legs form a planar base with the interface at the center thereof. The hypotenuses of the two component prisms form an entrance face and an exit face. When a collimated light source is directed at the entrance face and a reflecting mirror in a reference plane is parallel to the base, which is the same as normal to the interface, zero order interference fringes are seen at the exit face. This phenomenon is due to cancellation at the prism interface of beams which emanated from the source, were split into two rays at the interface, traveled along exactly equal optical path lengths and were recombined at the interface. When the mirror surface is not in the reference plane, the rays of a split beam do not travel exactly equal optical path lengths, and appearance of zero order interference fringes is affected. In the following description, the terms angular displacement or misalignment denote angular displacement or misalignment with respect to the reference plane. Various uses of this phenomenon have been made to provide interferometers which measure the angular displacement of a reflecting surface with respect to the reference plane. For example, U.S. Pat. No. 3,285,124, issued to Lovins, Nov. 15, 1966 discloses a beam-splitting prism modified by the addition of a tapered layer at the interface such that a zero order interference fringe translates along the interface with angular displacement of the reflecting surface. U.S. Pat. No. 2,880,644 to Brockway et al. issued Apr. 7, 1959 uses such a compound prism with one component prism rotated with respect to the other (very slightly) about an axis normal to the interface and prism apex. A plurality of interference fringes are seen when the mirror is aligned. Additional interference fringes rotated with respect to the original fringes are produced in response to misalignment. The observer must analyze the appearance of a number of fringes. These and other known prior systems have been limited to 0.01 arc-second of resolution of angular misalignment.
It is also necessary to provide a practical design for a beam splitter-reflector in an interferometer. Since in high resolution applications beam splitter-reflectors must be manufactured within exceedingly small tolerances (e.g. a fraction of an arc second), they cannot simply be machined according to blue prints. Generally they are ground or otherwise processed on an optical bench until there is an empirical indication (e.g. appearance of interference fringes) that the desired dimensions have been obtained. What on paper may appear to be simple modifications to a Koester prism may be extrememly difficult to manufacture.